1. Field of the Invention
The present invention generally relates to an optical scanner and an image forming apparatus, and more particularly to an optical scanner that can be preferably used as a writing system for a digital copier, a laser printer, a laser plotter and a facsimile, and an image forming apparatus that can be preferably used as a multi-color image forming apparatus for forming a color image by superposing color toner images.
2. Description of the Related Art
In an image forming apparatus for forming an image in accordance with Carlson process, a latent image forming process, a developing process and a transferring process are conducted in response to rotation of a photoconductor drum. If the photoconductor drum has an eccentric rotational axis or a drive motor for rotating the photoconductor drum has speed variations, the latent image forming process through the transferring process cannot be completed at a uniform time. As a result, a pitch irregularity, that is, an irregularity of intervals (scanning pitches) between individual optically-written line images, arises in the sub-scanning direction of a transferred image, resulting in a density irregularity.
In a tandem type color image forming apparatus, a plurality of photoconductor drums are arranged along the shift direction of a transferred member. The tandem type color image forming apparatus forms a multi-color image or a full-color image by transferring and superposing different color images formed on the photoconductor drums on the transferred member sequentially. If the pitch irregularity is caused in the tandem type color image forming apparatus, there is a risk that the individual superposed toner color images may be misaligned to each other. In this case, a color displacement or a color change is generated in the formed multi-color image or the formed full-color image, resulting in degradation of image quality.
In addition, a pitch irregularity arises in the transferring part because of variations of shift speeds of a transfer belt as a transferred member and a carrier belt to carry transferred paper. As a result, the density irregularity, the color displacement and the color change are caused. This pitch irregularity is caused by eccentricity and rotational speed variations of drive motors for rotating the transfer belt and the carrier belt.
In order to overcome problems such as the density irregularity, the color displacement and the color change, therefore, it is necessary to eliminate rotational speed variations of a drive motor for driving a photoconductor drum and overcome eccentricity and rotational speed variations of a drive motor for driving the transfer belt or the carrier belt. However, it is impossible to totally eliminate criteria of stable machining and load changes of transmission systems. As a result, it is impossible to totally eliminate the above-mentioned eccentricity and the rotational speed variations.
In the tandem type color image forming apparatus, if optical scanners fail to uniformly align resist positions of individual color latent images on the corresponding photoconductor drums with high accuracy, there is a risk that the produced color image may include color displacement and color change. Additionally, when the optical scanners write latent images by individual scanning lines, there is another risk that the scanning lines may have different slopes from each other. Furthermore, if the scanning lines are curved in degrees different from each other, the color displacement and the color change appears similarly.
Japanese Laid-Open Patent Application No. 08-014731 discloses a method for eliminating influences due to a rotational irregularity of a photoconductor drum. In this disclosed method, a time period from latent image forming process to the transferring process is set as an integral multiple of the period of the rotational irregularity. As a result, since the phase of a periodically varying position difference at time of the latent image formation coincides with the phase of a periodically varying position difference at time of the transfer, it is possible to cancel the influences.
Japanese Laid-Open Patent Application No. 10-197810 discloses a method for dynamically controlling an optical scanner. In this disclosed method, periodical emergence of the pitch irregularity is focused. When the pitch irregularity is detected, a correction mirror is shifted corresponding to rotation of a photoconductor drum.
The pitch irregularity appears in an image as syntheses of low frequency factors and high frequency factors wherein the low frequency factors result from rotational irregularities of drive motors for a photoconductor drum, a transfer belt and a carrier belt and, on the other hand, the high frequency factors result from engagement of gears of transmission systems. As an image is required to have higher image quality, more accurate gears are used for the transmission systems and there is a stronger tendency that the photoconductor drum and the transfer belt are directly driven by the drive motors so that malfunctions between the transmission systems cannot affect image degradation. Also, by increasing the inertial force by means of a flywheel, the high frequency factors can be reduced.
However, it is impossible to prevent the influences exerted by the low frequency factors due to the load variations and others that are involved in the eccentricity and assembly differences in association with fabrication precision of parts thereof. Therefore, it is more important to suppress the low frequency factors.
Especially in the tandem type color image forming apparatus, the pitch irregularity period with respect to the sub-scanning direction, which results from variations of transferring timings of toner images, has a different phase or a different amplitude for each image. According to the above-mentioned methods, therefore, it is impossible to arrange dot positions of the individual images with high accuracy.
In order to adapt resist positions of individual latent images, a difference between the resist positions is detected through images recorded on a transferred member. Additionally, the resist positions with respect to the sub-scanning direction are aligned by adjusting writing timings.
On the other hand, there are some correction methods for correcting the curvature and gradient of a scanning line. Japanese Patent No. 3049606 discloses a correction method for correcting the curvature and gradient of a scanning line by curving a reflection mirror disposed in an optical path and inclining the reflection mirror of a surface parallel to a transfer surface. Japanese Laid-Open Patent Application No. 11-064758 discloses a correction method by changing heights of the optical axes of a part of lenses constituting an image forming optical system. Japanese Laid-Open Patent Application No. 10-268217 discloses a correction method by forcing the body of a lens to be curved. Japanese Laid-Open Patent Application No. 11-153765 discloses a correction method for rotating a part of lenses constituting an image forming optical system in the directions of the optical axes of the lenses.
In recent years, a resin-molded lens has been used as an image forming part of an optical scanner. Such a lens has some advantages in that, for example, a resin-molded lens can be shaped to have complicated surface shape at a reasonable cost. In contrast, the lens has disadvantages in that even if the curvature and gradient of a scanning line is initially adjusted, harmful curvature and gradient may arise because of deformation of the lens body due to temperature variations of the environment.
In particular, when a resin-molded lens is disposed away from a deflecting part, the stiffness of the resin-molded lens tends to be small because the lens is longer with respect to the main scanning direction. As conventional methods, if one surface of the lens with respect to the sub-scanning direction is in contact with something to maintain the orientation thereof, there is a risk that the lens body may be deformed because the lens is retained in a condition where stress such as a warp and a torsion is imposed on the lens.
If the curvature of a scanning line is attempted to be corrected by forcibly curving a resin-molded lens in accordance to Japanese Laid-Open Patent Application No. 10-268217, there is a risk that the surface shape of the lens is deformed by stress concentration. If an optical beam is deflected toward the refractive index of interior of a lens whose distribution is centered at the optical axis thereof with respect to the sub-scanning direction in accordance with Japanese Laid-Open Patent Application No. 11-064758, there is a risk that the diameter of a beam spot is not uniform on a photoconductor drum.
Additionally, a base member for retaining a resin-molded lens has thermal conductivity different from the outer atmosphere, resulting in a temperature difference between the lens surface in contact with the base member and the opposite lens surface exposed to the outer atmosphere. As a result, there is a problem that the lens body is deformed and curved over time.
In addition, a resist difference is conventionally detected based on a resist difference detection pattern recorded on a transferred body and is adjusted with respect to the sub-scanning direction by changing the writing timing as disclosed in Japanese Patents No. 3049606 and No. 3078830.
Japanese Laid-Open Patent Application No. 11-064769 discloses a method of correcting a scanning position by using a galvanometer mirror, which is disposed between an illuminant and an image forming optical system, to incline the optical axis of an optical beam in the sub-scanning direction.
Japanese Patent No. 2672313 discloses a method of correcting a scanning position by parallel-shifting a folding mirror.
When a multi-color image forming apparatus, in which a plurality of image forming stations are disposed along the carrying direction of a transferred member, forms a color image by superposing individual simple color images, color displacement or color change appears in the formed color image if resist positions, where individual latent images formed by the image forming stations are transferred, do not accurately coincide with each other.
However, even if an optical scanner is initially adjusted to correct a difference between scanning positions, which can cause a difference between resist positions, between individual image forming stations, there is a risk that temperature variations may deform the housing of the optical scanner and cause variations of the refractive index of a scanning lens. Therefore, it is impossible to avoid variations of resist positions over time.
For this reason, although the above-mentioned difference between resist positions is periodically detected and corrected, it is impossible to align the resist positions at the writing head of an image uniformly. As a result, for example, if the transferred member does not move at a constant speed, a more significant color displacement and color change can appear at the writing end of the image.
A difference between resist positions is conventionally corrected by adjusting the writing timings of the individual image forming stations. A synchronizing detection signal of each surface of a polygon mirror is used as triggers to determine the writing timings. For this reason, the writing timings cannot be adjusted for less than the sub-scanning pitch, which corresponds to the recording density. As a result, there is a risk that a difference between the resist positions, whose maximum size is a half of the sub-scanning pitch, may appear on an image.
Additionally, if there is a speed difference between the transferring position and the detecting position due to speed variations of the transferred body, there is a risk that a detected resist difference may contain an error. Furthermore, if the synchronizing detection sensor is not accurately positioned due to thermal expansion of the housing, the synchronizing detection signal is generated at various timings. The detected resist difference and the synchronizing detection signal is used as references of the feedback control of the optical scanner. Thus, it is impossible to align the resist positions uniformly based on such inaccurate references even if the adjusting process is properly operated.
In addition, a recent increase in the operational speed of color image forming apparatuses has realized practical use of color digital copiers, color laser printers and so on. In a four-drum tandem type color image forming apparatus, for example, four photoconductor drums are arranged in the carrying direction of record papers. A plurality of optical scanning systems corresponding to the individual photoconductor drums simultaneously expose the photoconductor drums so as to form latent images. These latent images are made visible by using different color developers such as yellow, magenta, cyan and black. Then, these developed simple color images are sequentially superposed and transferred onto a same record paper so as to form a full-color image.
Alternatively, a one-drum type image forming apparatus has only one photoconductor drum. In such a one-drum type image forming apparatus, the photoconductor drum is rotated as many times as the number of prepared colors. For each rotation, a latent image forming process (exposing process), a developing process and a transferring process are performed for the photoconductor drum, and then the resulting visible simple color images are superposed and transferred onto a same record paper so as to form a full-color image. As another embodiment of the one-drum type color image forming apparatus, after the visible simple color images are formed, the visible simple color images may be temporarily superposed onto an intermediate transferred member and then transferred onto the record paper.
The four-drum tandem type color image forming apparatus has an advantage compared to the one-drum type color image forming apparatus in that the four-drum tandem type color image forming apparatus can produce a color image and a monochrome image at a same speed. Thus, the four-drum tandem type color image forming apparatus is more suitable to high speed printing. In contrast, since the four-drum tandem type color image forming apparatus contains four optical scanning systems to expose four photoconductor drums, the four-drum tandem type color image forming apparatus tends to have a greater size and it is necessary to reduce the size thereof. Additionally, the four-drum tandem type color image forming apparatus has another problem in that color displacement may occur when individual color toner images corresponding to the four photoconductor drums are superposed and transferred onto a record paper.
In particular, the color displacement can be caused with respect to the sub-scanning direction by the following factors.                A speed variation of a photoconductor with respect to the circumferential direction (sub-scanning direction).        A speed variation of an intermediate transferred member with respect to the circumferential direction (sub-scanning direction).        A position difference between photoconductors.        A position difference of optical spots between optical scanning systems.        If a plurality of optical beams are simultaneously used to write latent images on individual photoconductor drums, there is a risk that misalignment with respect to the sub-scanning direction may be caused corresponding to the number of prepared optical beams because a polygon scanner is not rotated synchronously with the photoconductor drums in general.        
The following conventional methods for suppressing such color displacement are presented.
Japanese Laid-Open Patent Application No. 2001-133718 discloses an invention that can make scanning lines on individual photoconductor drums coincide with each other by adjusting positions of the individual scanning means or the housings thereof relative to the photoconductor drums. According to this invention, however, the adjustment mechanism becomes complicated and it takes a large amount of adjustment time. In addition, since the heavy housings are adjusted, it is difficult to correspond to changes over time due to temperature variations. Also, it is difficult to accurately correct color displacement during printing operation or color displacement due to variations of the environment.
Japanese Laid-Open Patent Application No. 2001-100127 discloses a method for controlling the position of an optical beam with respect to the sub-scanning direction by using a galvanometer mirror. According to this disclosed method, however, since the galvanometer mirror is too sensitive for the purpose of controlling the optical beam position of with respect to the sub-scanning direction, the galvanometer mirror is highly influenced by external vibrations. In order to obtain a better beam spot diameter, it is necessary to satisfy high surface accuracy (about four times of a transmission surface).
Japanese Laid-Open Patent Application No. 10-239939 discloses a color image forming apparatus that includes color displacement correction means. In this color image forming apparatus, an optical beam for first writing an image on a photoconductor is selected among a plurality of optical beams based on a phase relation between a reference intermediate transferring signal and a line synchronizing signal so as to adjust starting positions for writing individual color images with respect to the sub-scanning direction. According to this color image forming apparatus, however, it is difficult to correct color displacement smaller than one line. For instance, if individual simple color images are written at 600 dpi (dots per inch), there is a risk that a full-color image generated from the individual color images may have color displacement of at least more than 42 μm.